Recently, as a result of the spread of internet communications, etc., the quantity of information communicated via communications lines has been increasing, and there has been a demand for improved productivity in optical filters used in the optical fibers that constitute the mainstream of current communications lines. The filter chips that constitute the source of such optical filters are obtained by using a dicing saw, etc., to cut filter members in which a filter film is formed on the surface of a glass substrate to the required dimensions.
The formation of such filter films on glass substrates is accomplished inside a film forming apparatus such as a sputtering apparatus, etc. Various types of such film forming apparatuses are known; apparatuses in which film formation is performed with a plurality of rectangular glass substrates attached to a rotating substrate holder include apparatuses in which the portion of the filter film present on the filter member 90 being manufactured that has good characteristics, i.e., a filter film 91 with good characteristics, is formed in a circular arc form pattern as shown in FIG. 21. Furthermore, a plurality of the desired filter chips 92 can be obtained by cutting the filter member 90 thus obtained so that the above-mentioned filter film 91 with good characteristics is divided into a checkerboard pattern as shown in FIG. 21.
In recent years, furthermore, wavelength division multiplexing (WDM communications system: wavelength division multiplexing communications system) which is a technique that easily allows an increase in the communications capacity among optical communications techniques has begun to be used. In the field of this WDM communications system, optical multi-layer film filters used as optical filters that split light in the near infrared region used for communications into a plurality of narrow wavelength regions, and synthesize the light of this plurality of wavelength regions, have attracted attention.
However, in cases where filter members are cut so that the filter film with good characteristics that is present in a circular arc form pattern is split into a checkerboard pattern as described above, there are many wasted portions that cannot be cut out into filter chips even though the characteristics are good, so that the number of filter chips obtained is small, and the yield is poor. Furthermore, in cases where the filter film that is formed on the glass substrate is an optical multi-layer film of the above-mentioned type, this optical multi-layer film is thicker than a conventional filter film; as a result, there have been cases in which chipping, cracking or peeling of the film occurs when cutting is performed using a dicing saw. In the past, therefore, it has been necessary to use a working method in which an optical multi-layer film is formed on glass substrates that have been cut to the required dimensions beforehand, so that the productivity of optical multi-layer film filters has not been very good.